Fuel oil is a fraction obtained from petroleum distillation, either as a distillate or a residue. Broadly speaking, fuel oil is any liquid petroleum product that is burned in a furnace or boiler for the generation of heat or used in an engine for the generation of power, except oils having a flash point of approximately 40 °C (104 °F) and oils burned in cotton or wool-wick burners. In this sense, diesel is a type of fuel oil. Fuel oil is made of long hydrocarbon chains, particularly alkanes, cycloalkanes and aromatics. The term fuel oil is also used in a stricter sense to refer only to the heaviest commercial fuel that can be obtained from crude oil, heavier than gasoline and naphtha
Heavy fuel oil used in marine main diesel engines is a residue from crude oil refining. Because of the properties of heavy fuel oil it is required be kept at a high storage temperature and for usage. Despite this heavy fuel combustion products remain high in NOx, Sox, and CO2 in the exhaust gases.
as a marine engine fuel because of its cheapness, but it does contain a lot of impurities and must be kept at a high temperature both during storage and use in ships main engines.